1. Field of the Invention
The present invention relates to an object proximity detector and an object position detector. In particular, the present invention relates to a position detector with multiple transmission gates connected to the input and output of an oscillator and sensor plates connected to the transmission gates.
2. Description of the Related Art
Modern proximity detectors have been used to indicate when one object is close to a detector and to measure how far away one object is from the detector. Capacitive sensors and inductive sensors often are used in proximity detectors. Capacitive proximity sensors translate the variation of capacitance to a binary signal, to determine whether that effective capacitance has been exceeded. The variation of capacitance relates to a distance between an object and the sensor plate/plates. There are a variety of well-known ways of measuring capacitance between the sensor plates. One way is to feed an AC signal to an amplifier through sensor plates and to measure the variation of amplitude of AC signal at the output of amplifier. The technology is used in U.S. Pat. No. 5,374,787, U.S. Pat. No. 5,495,077, U.S. Pat. No. 5,841,078, to Robert J. Miller et al., U.S. Pat. No. 5,914,465, U.S. Pat. No. 6,239,389B1, to Timothy P. Allen et al., U.S. Pat. No. 6,028,271, U.S. Pat. No. 6,610,936B2, to David W. Gillespie et al. The system uses this technology consists of a lot of analog circuit, such as amplifier, filter, minimum selector, subtract circuit, sample/hold and A/D converter. The chip size of analog circuit is much bigger than that of digital circuit in an integrated circuit, and is not cost effective. The technology used in U.S. Pat. No. 6,452,514 B1 and U.S. Pat. No. 6,466,036 B1, to Harald Philipp is a charge transfer circuit. In this circuit, an AC voltage source is applied to one plate of the sensor and fed into a signal processor through the other plate of the sensor. The signal processor consists of charge transfer circuit, integrator and voltage measurement circuit. Also a lot of analog circuits used in the above technology. Besides the analog circuits, a lot of high speed analog switches are used. The clock feed-through caused by the parasitic capacitance of analog switches will cause the distortion of the signal. David W. Caldwell et al in U.S. Pat. No. 5,572,205, teaches a touch control system. In this system also apply an AC voltage source to one plate of the sensor and fed to a signal processor through the other plate of the sensor. The signal processor consists of analog circuits, such as peak detector, amplifier and A/D converter. The interference from RF signal will add to the peak detector directly, and caused the error detection of the system. One of the other ways is to connect the sensor plates at the input of an oscillator. The variation of capacitance at the input of oscillator will cause the variation of oscillator frequency. By detection of variation of frequency, the proximity of an object to the sensor plates will be detected. The technology is used in U.S. Pat. No. 6,583,676B2, to Christoph H. Krah et al. The frequency of the oscillator depends on the parameters of process and the power supply voltage. The proximity detectors require frequent calibration to compensate those variations. As described in the patent, the prior art uses two capacitors and a transistor to emulate when the sensor plates of the oscillator is in the proximity or not in the proximity by an object. Because the capacitors and transistor are built in the integrated circuit, the sensitivity of the proximity detector is difficult to be changed and also is difficult to be programmed externally. In order to avoid the frequent calibration of the object proximity sensor, we need to design an oscillator such that the dependence of frequency on process parameters and power supply is reduced to a minimum. The invention is to add addition circuit to the system to compensate the process dependence of oscillator frequency. One of the RC oscillator which is used commonly in the prior art is described in FIG. 1. This circuit consists of three inverters, 101, 102, 103, a 10; resistor 104, a capacitor 106, a pair of sensor plates 105 with capacitance Cs. The first stage, 101, is an inverter with Schmitt trigger input. A resistor 104 in the feedback loop of the oscillator is used as the charging/discharging element of the circuit. The frequency of the oscillator is determined by the resistor 104 and the capacitors 105, 106. The waveform of the circuit is shown in FIG. 2. Where VTR2 and VTR1 are two transfer voltages of the Schmitt trigger input inverter 101. During the charging cycle of the circuit, when the voltage at the input of the inverter 101 arrives at VTR2, the output of the inverter 103 changes state and the circuit starts to a discharge cycle. The voltage at the input of the first inverter 1101 sweeps between VTR2 and VTR1. The period of the oscillator is proportional to R(Cs+C) (VTR2−VTR1)/(Vcc−(VTR2+VTR1)/2)+dt, where dt is the propagation delay of the inverters, and Vcc is the power supply voltage. From the equation, we know, the frequency depends a lot on the transfer voltages VTR2 and VTR1. If the circuit is designed by CMOS process, the voltage gap, VTR2−VTR1, depends a lot on the threshold voltages of PMOS and NMOS transistors. If the power supply voltage decreases, VTR2−VTR1 will decreases and dt will increase. Because the propagation delay is very small in an integrated circuit, the increasing of dt is not enough to compensate the decreasing of VTR2−VTR1.